2,841 research outputs found
Basis Set Incompleteness Errors in Fixed-Node Diffusion Monte Carlo Calculations on Noncovalent Interactions
No full textBasis set incompleteness error (BSIE) is a common
source of error in quantum chemistry calculations, but it has not
been comprehensively studied in fixed-node Diffusion Monte Carlo
(FN-DMC) calculations. FN-DMC, being a projection method, is
often considered minimally affected by basis set biases. Here, we
show that this assumption is not always valid. While the relative
error introduced by a small basis set in the total FN-DMC energy is
minor, it can become significant in binding energy (Eb) evaluations
of weakly interacting systems. We systematically investigated BSIEs
in FN-DMC-based Eb evaluations using the A24 data set, a well-
known benchmark set of 24 noncovalently bound dimers. We found
that BSIEs in FN-DMC evaluations of Eb are indeed significant
when small localized basis sets, such as cc-pVDZ and cc-pVTZ, are
employed. Our study shows that the aug-cc-pVTZ basis set family strikes a good balance between computational cost and BSIEs in
the Eb calculations. We also found that augmenting the basis sets with diffuse orbitals, using counterpoise correction, or both,
effectively mitigates BSIEs, allowing smaller basis sets such as
aug-cc-pVDZ to be used
Benjamin Karim: Remembering Malcolm
No full textBenjamin Karim (July 14, 1932 - August 2, 2005) was an African American Muslim minister, author, and orator. He was born Benjamin Goodman, in Suffolk, VA., the son of Wilbur Bryant and Mary Goodman. Young Goodman was a U.S. Air Force veteran of the Korean War. After serving in the military, Goodman worked as a recording engineer with a record company when he first heard Malcolm X speak in 1957, and his experience caused him to convert to the Nation of Islam (NOI), cease many secular activities, change his diet, and become sober.
A dedicated student of African and African American history, Karim re-educated himself and over the next seven years, adopted the name Benjamin 2X, and became one of the closest aides to Malcolm X. He supervised an educational program at the NOI temple in Harlem, and stood in for Malcolm X at some events held around the United States
Systematic discrepancies between reference methods for noncovalent interactions within the S66 dataset
No full textThe accurate treatment of noncovalent interactions is necessary to model a wide range of applications, from molecular crystals to surface catalysts to aqueous solutions and many more. Quantum diffusion Monte Carlo (DMC) and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)] are considered two widely trusted methods for treating noncovalent interactions. However, while they have been well-validated for small molecules, recent work has indicated that these two methods can disagree by more than 7.5 kcal/mol for larger systems. The origin of this discrepancy remains unknown. Moreover, the lack of systematic comparisons, particularly for medium-sized complexes, has made it difficult to identify which systems may be prone to such disagreements and the potential scale of these differences. In this work, we leverage the latest developments in DMC to compute interaction energies for the entire S66 dataset, containing 66 medium-sized complexes with a balanced representation of dispersion and electrostatic interactions. Comparison to previous CCSD(T) references reveals systematic trends, with DMC predicting stronger binding than CCSD(T) for electrostatic-dominated systems, while the binding becomes weaker for dispersion-dominated systems. We show that the relative strength of this discrepancy is correlated to the ratio of electrostatic and dispersion interactions, as obtained from energy decomposition analysis methods. Finally, we have pinpointed model systems: the hydrogen-bonded acetic acid dimer (ID 20) and dispersion-dominated uracil–cyclopentane dimer (ID 42), where these discrepancies are particularly prominent. These systems offer cost-effective benchmarks to guide future developments in DMC, CCSD(T), as well as the wider electronic structure theory community
Accurate and efficient machine learning interatomic potentials for finite temperature modelling of molecular crystals
No full textAs with many parts of the natural sciences, machine learning interatomic potentials (MLIPs) are revolutionizing the modelling of molecular crystals. However, challenges remain for the accurate and efficient calculation of sublimation enthalpies - a key thermodynamic quantity measuring the stability of a molecular crystal. Specifically, two key stumbling blocks are: (i) the need for thousands of ab initio quality reference structures to generate training data; and (ii) the sometimes unreliable nature of density functional theory, the main technique for generating such data. Exploiting recent developments in foundation models for chemistry and materials science alongside accurate quantum diffusion Monte Carlo benchmarks, offers a promising path forward. Herein, we demonstrate the generation of MLIPs capable of describing molecular crystals at finite temperature and pressure with sub-chemical accuracy, using as few as ∼200 data structures; an order of magnitude improvement over the current state-of-the-art. We apply this framework to compute the sublimation enthalpies of the X23 dataset, accounting for anharmonicity and nuclear quantum effects, achieving sub-chemical accuracy with respect to experiment. Importantly, we show that our framework can be generalized to crystals of pharmaceutical relevance, including paracetamol and aspirin. Nuclear quantum effects are also accurately captured as shown for the case of squaric acid. By enabling accurate modelling at ambient conditions, this work paves the way for deeper insights into pharmaceutical and biological systems
PLAYER'S CIGARETTES BENJAMIN DISRAELI
No full textCigarette card depicting politician and author Benjamin Disraeli as a young man with dark hair, black hat in right hand, dark jacket, orange waistcoat, red trousers w. gold stripe, cane in left hand. Rev: A SERIES OF 25 DANDIES FROM PAINTINGS BY CHROSTOPER CLARK, R.I. No. 22 DIZZY in 1826 ... PLAYER'S CIGARETTES. ISSUED BY JOHN PLAYER & SONS BRANCH OF THE IMPERIAL TOBACCO CO. OF GREAT BRITAIN & IRELAND, LTD 3 1/8 X 2 1/2 in.; 7.8 x 6.2 mm.Digital imagedigitize
Self-assembly and DNA binding of the blocking factor in X chromosome inactivation
Get PDFX chromosome inactivation (XCI) is the phenomenon occurring in female mammals whereby dosage compensation of
X-linked genes is obtained by transcriptional silencing of one of their two X chromosomes, randomly chosen during
early embryo development. The earliest steps of random X-inactivation, involving counting of the X chromosomes and
choice of the active and inactive X, are still not understood. To explain "counting and choice," the longstanding
hypothesis is that a molecular complex, a "blocking factor" (BF), exists. The BF is present in a single copy and can
randomly bind to just one X per cell which is protected from inactivation, as the second X is inactivated by default. In
such a picture, the missing crucial step is to explain how the molecular complex is self-assembled, why only one is
formed, and how it binds only one X. We answer these questions within the framework of a schematic Statistical
Physics model, investigated by Monte Carlo computer simulations. We show that a single complex is assembled as a
result of a thermodynamic process relying on a phase transition occurring in the system which spontaneously breaks
the symmetry between the X’s. We discuss, then, the BF interaction with X chromosomes. The thermodynamics of the
mechanism that directs the two chromosomes to opposite fates could be, thus, clarified. The insights on the selfassembling
and X binding properties of the BF are used to derive a quantitative scenario of biological implications
describing current experimental evidences on "counting and choice.
Fore and Aft Channel Reconstruction in the TerraSAR-X Dual Receive Antenna Mode
No full textThe TerraSAR-X satellite is a high resolution synthetic aperture radar (SAR) system launched in June 2007 which provides the option to split the antenna in along-track direction and sample two physical channels separately. Modern SARs are equipped with active phased array antennas and multiple channels. In order to keep costs low TerraSAR-X uses the redundant receiver unit for the second channel such that fore and aft channel signals are combined by a hybrid coupler to form sum and difference channel data. The dual receive antenna mode can either be used to acquire along-track interferometric data or to acquire signals with different polarizations at the same time (Quad pol). Fore and aft channel reconstruction is necessary if ground moving target indication algorithms such as the displaced phase center antenna technique or along-track interferometry shall be applied, and in order to separate the horizontally and vertically polarized received signal components. The proposed approach uses internal calibration pulses from different calibration beams in order to estimate and compensate the hardware impact. The theoretical framework together with the results from the experimental data evaluation for the fore and aft channel reconstruction of the TerraSAR-X dual receive antenna mode are presented. The impact of the receive hardware transformation matrix estimation accuracy on errors in the reconstructed fore and aft channel image data is studied, and
first examples on the ground moving target indication capability of the TerraSAR-X dual receive antenna mode are given
Cooperative CO2 capture via oxalate formation on metal-decorated graphene
No full textCO2 capture using carbon-based materials, particularly graphene and graphene-like materials, is a promising strategy to deal with CO2 emissions. However, significant gaps remain in our understanding of the molecular-level interaction between CO2 molecules and graphene, particularly in terms of chemical bonding and electron transfer. In this work, we employ random structure search and density functional theory to understand the adsorption of CO2 molecules on Ca-, Sr-, Na-, K-, and Ti-decorated graphene surfaces. Compared to the pristine material, we observe enhanced CO2 adsorption on the decorated graphene surfaces. Particularly on group 2 metals and titanium-decorated graphene, CO2 can be strongly chemisorbed as a bent CO2 anion or as an oxalate, depending on the number of CO2 molecules. Electronic structure analysis reveals that the adsorption mechanism involves a charge transfer from the metal adatom to the adsorbed CO2. Overall, this study suggests that reducing CO2 to oxalate on group 2 metals and titanium-decorated graphene surfaces is a potential strategy for CO2 storage
An Investigation in Applying Image Retrieval Techniques to X-Ray Engineering Pictures
No full textUsing image retrieval techniques in analysing Non-destructive testing reults is a new challenge in both
computing science and engineering applications. Objective of this research is to develop an image retrieval system
to analyse X-ray images for welding industry. The content based image retrieval has been used in this investigation,
particularly in feature vector paradigm and similarity as well as detailed analysis towards single defects. It is found
that X-ray images can be digitally analysed qualitatively and quantitatively easily. It concludes that the use of
existing CBIR techniques can provide a platform to quickly develop new image analysis tools
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